1. Field of the Invention
This invention relates to superconductor materials. More particularly, this invention relates to a method of forming shaped superconductor materials by electrophoretic deposition of previously formed superconductor particulate coated with a fusible binder followed by fusion of the binder, while optionally orienting the particulate in an electrostatic field, and subsequent removal of the binder and sintering of the superconductor particles.
2. Description of the Related Art
Since the discovery of superconductivity in 1911, the phenomena of a material being able to conduct electricity with almost no resistance when the material is cooled to a temperature approaching absolute zero (0.degree. K.) has remained an interesting scientific curiosity having few applications which would justify the expense of maintaining the necessary liquid helium cooled system.
Recently, however, superconducting ceramic materials have been produced which exhibit this phenomena at much higher temperatures, e.g., 35.degree. K., and, in some cases even higher than the boiling point of liquid nitrogen which boils at about 77.degree. K. The ability to produce superconductivity in a material cooled by liquid nitrogen completely changes the economics which have previously restricted the applications to which superconductivity could be applied.
These new ceramic materials are sometimes referred to as triple-layer perovskite compounds because of the crystallography of the resulting structure; or 1-2-3 compounds because of the atomic ratios of at least some of these ceramic superconductors which may, for example, comprise 1 atom of a rare-earth (Lanthanum series) element such as lanthanum or yttrium, 2 atoms of an alkaline earth metal such as barium or strontium, and 3 atoms of copper. The superconducting ceramic also contains from 6.5+ to 7- atoms of oxygen which is usually referred to as O.sub.(6.5+x) where x is greater than 0 and less than 0.5, resulting in a chemical formula such as, for example, YBa.sub.2 Cu.sub.3 O.sub.(6.5+x).
While the superconducting properties of such ceramic materials have been confirmed by demonstration of the Meissner effect wherein the superconductor, when cooled to superconducting temperature, will exhibit magnetic properties sufficient to levitate a magnet above the superconductor, such ceramic superconducting materials are deficient in some of the essential physical properties needed to permit fabrication and practical usage of structures from such materials.
Most notable of these deficiencies is the extreme brittleness and poor mechanical strength of the superconducting ceramic structures which inhibits formation of shaped structures, e.g., coils or wires therefrom. Such superconducting ceramic materials also show evidence of extreme microcracking which is a further indication of brittleness and would also effect critical current density J.sub.c.
In the fabrication of the previously known prior art superconductors, i.e., metal alloy superconductors which only exhibit superconducting characteristics at low absolute temperatures .ltoreq.25.degree. K., Barber U.S. Pat. No. 3,466,237 describes the electrophoretic deposition of a powder layer of one of the metals which will form the superconducting alloy on a substrate comprising another metal in the superconducting alloy. The coated material is then heated sufficient to cause the two metals to react to form a layer of the superconducting alloy.
Great Britain Patent No. 1,151,492 describes a somewhat similar process for making conventional superconducting alloys, i.e., metal alloys exhibiting superconductivity at low absolute temperatures (T&lt;25.degree. C.), in which both metals which will ultimately form the superconducting alloy are electrophoretically deposited as powders on a substrate after which the coated substrate is heated to form the superconducting alloy.
Production of superconductors using electrophoresis is also apparently described by Korobko et al in Sverkhprovodimosti, Leningrad, 26-28 Sent., 1983, L. Issue, pp. 88-89, based on the English translation of the title as "Production of Superconducting Coatings by Electrophoresis".
The use of electrophoretic deposition techniques to form coatings which may, upon subsequent processing, exhibit superconducting properties is certainly of interest due to the close packing of materials which are deposited electrophoretically.
However, in the formation of the prior art metal alloy superconductors, subsequent processing of the metal alloy, such as to mold or fabricate it into a desired shape, would not involve the same problems as attempting to shape the recently discovered ceramic or metal oxide type superconductor materials because metal alloys are generally malleable, as opposed to the characteristic brittleness of the latter.
Thus, if the steps of electrophoretic deposition followed by heating to form the superconductor film, as taught in the prior art for forming superconducting metal alloys, were to be used in forming the high temperature superconducting ceramic or metal oxide films, the resulting film or coating would be too brittle to permit any subsequent shaping or fabricating.
Conversely, if the more recently discovered metal oxide or ceramic superconducting materials were to be formed first, then electrophoretically deposited in particulate form on a substrate, and then fabricated or shaped without heating the coated substrate to sinter the deposited particles together, portions of the deposited coating would become dislodged during such fabrication or shaping steps.
It would, therefore, be desirable to be able to electrophoretically deposit on a substrate or support previously formed particulate superconducting material of the ceramic or metal oxide type and to be able to then fabricate or form the coated substrate or support into a desired shape, prior to sintering of the particulate mass comprising the coating, without significant risk of loss of portions of the deposited coating during such fabrication.